Excretion and Homeostasis
This domain covers removal of nitrogenous waste and the maintenance of a stable internal environment. The exam tests nephron structure and urine formation, the excretory work of skin, lungs and liver, plant excretory products, and homeostatic control of temperature, water and blood sugar.
Human excretory system
- Organs: kidneys, ureters, urinary bladder, urethra. Each kidney has about a million nephrons.
- Ultrafiltration in the glomerulus and Bowman's capsule forces water, glucose, amino acids, salts and urea into the tubule; blood cells and protein are retained.
- Selective reabsorption in the proximal tubule returns glucose, amino acids, most salts and much water; normally all glucose is reabsorbed.
- Urine contains water, urea, uric acid, creatinine and salts. Kidney failure may need dialysis.
Other excretory organs
- Liver: deamination of surplus amino acids forms urea and bile pigments.
- Lungs: carbon dioxide and water vapour. Skin: sweat with salts and urea.
- Plants: stomata and lenticels lose gases; wastes are stored as gums, resins, latex, alkaloids and calcium oxalate crystals, or secreted through hydathodes.
Homeostasis
- Temperature: the hypothalamus coordinates sweating, vasodilation and vasoconstriction, and shivering.
- Water balance: antidiuretic hormone increases water reabsorption in the kidney tubules.
- Blood sugar: insulin lowers glucose, glucagon raises it.
Worked example
On a hot day body temperature rises. The hypothalamus triggers vasodilation of skin arterioles and increased sweating. Evaporation of sweat removes latent heat and cools the blood, and the temperature returns towards normal; on a cold day the same centre triggers vasoconstriction and shivering instead.
Traps that cost marks
- Glucose in urine signals that reabsorption has failed, as in diabetes mellitus.
- Antidiuretic hormone increases water reabsorption, so more hormone means less urine.
- Egestion of faeces is not excretion; it removes undigested matter.